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A senior Bush administration health official and a critic are publicly debating a pending government decision on whether to vaccinate the U.S. population against smallpox.

An expert advisory panel, the Advisory Committee on Immunization Practices (ACIP), last month recommended to Health and Human Services Secretary Tommy Thompson against mass vaccination. Instead, the panel favored vaccinating certain U.S. healthcare workers and using selective vaccination through a containment strategy in the event of an outbreak. Thompson's decision is pending.

The debate centers on whether that "ring vaccination strategy," which involves vaccinating an infected person's immediate contacts and household members, would be sufficient to contain the spread of the disease in the event of a smallpox attack.

The strategy was used to eradicate smallpox globally during the 1960s and 1970s and is considered beneficial since it minimizes exposure to the smallpox vaccine's side effects, estimated to kill three people per million vaccinations.

A prominent proponent of that ring strategy is D.A. Henderson, who led the successful eradication effort and is now Thompson's principal science advisor for public health emergency preparedness.

"Let's say we've got a case, and that individual is going to be in contact with a lot of people, and what you want to do is to vaccinate those contacts. If you get to them with in the first two to three days, you can prevent the disease," Henderson said, speaking on a panel Friday intended to provide information on the pending decision.

The ring strategy was challenged earlier this month when Yale University professor Edward Kaplan co-authored a study suggesting a mass vaccination would be a better alternative to deal with a terrorist attack using smallpox.

"Dr. Henderson is treating a bioterror attack as if it were a historical, natural smallpox outbreak. It is not wise to prepare for a smallpox bioterror attack-where terrorists are trying as hard as they can to kill as many of us as possible-in this fashion," Kaplan said.

Specifically, Henderson and Kaplan differ on how quickly the disease would spread through the population.

Kaplan's study, modeling a hypothetical attack on a major city with an initial 1,000 people infected, projected the ring strategy would allow 367,000 people to become infected and 110,000 to die before eliminating the disease after 350 days. Mass vaccinations administered quickly after an attack, it estimated, would allow 1,830 cases and 560 deaths over 115 days.

Henderson, in an interview Friday, argued Kaplan's study gave short shrift to the ring strategy by using extreme assumptions about the spread of the disease. He criticized the model's assumption that a single infected person would come into sufficiently close contact with 50 people before the initial case is detected.

"You've got to see some of these patients ... These are people that are sick, they're not wandering around the country."

Kaplan agreed in an interview that his analysis modeled a worst-case scenario, but argued such a strategy is preferable since terrorists would likely aim to cause the worst amount of damage.

"We believe that when planning a bioterror response policy, it is important to develop a robust approach that can succeed in situations much worse than history has provided," he said.

Kaplan said Henderson is too conservative about estimating the spread of the disease, saying data suggests infectious people could circulate in society.

Historically, he said, "It is true that more cases were transmitted within households or hospitals than by 'casual' transmission-but this does not mean that transmission in the workplace, or via casual contacts (e.g. on a bus, in a marketplace) was insignificant."

Kaplan cited a slide used by Centers for Disease Control and Prevention officials suggesting at least one-fourth of all smallpox transmissions in Europe did not occur in the home or hospital.

"This [was] in situations of natural outbreaks, and in populations with relatively high levels of immunity (due to past vaccination campaigns or survival from prior smallpox outbreaks), and in much less mobile populations than, say, New York City in the 21st century."

"Dr. Henderson's experience involved natural smallpox outbreaks in much less mobile populations that already had moderate to moderately high levels of immunity," he said.

Kaplan also cited a paper that appeared in the magazine Nature last fall, which provided mathematical models of historical smallpox outbreaks. The models assumed random interactions between infected and susceptible in the population.

Kaplan also faults the advisory panel for not addressing the probable state of readiness of U.S. authorities to respond to an attack and to contain it effectively using the ring strategy.

"The issue was not addressed because the panel did not perform any analysis of their own ring vaccination response plan. They simply assumed that ring vaccination would work via historical analogy," said Kaplan.

In recommending against mass vaccination, the panel weighed two primary risks, the side effects of mass vaccination and the probability of an attack.

The advisory committee concluded, "Under current circumstances, with no confirmed smallpox, and the risk of an attack assessed as low, vaccination of the general population is not recommended, as the potential benefits of vaccination do not outweigh the risks of vaccine complications."

The calculation should have included another factor, Kaplan said-U.S. response capabilities.

When weighing the consequences of mass vaccination versus those of an attack on a society that is not mass vaccinated, the latter "depends critically on what the response policy is" he said.

"The basic lesson is this," he writes, "If you harbor serious doubts regarding the ability of whatever response policy is employed to control the epidemic, then you should be more willing to vaccinate pre-attack. On the other hand, if you are highly confident that whatever response policy is employed can contain the epidemic, then the pressure to vaccinate before an attack is reduced."

ACIP Chairman John Modlin said he believes U.S. authorities would respond well if an outbreak occurs today.

"I would just point out that there are no guarantees in life, but the best estimate is that with communication, with knowledge of an outbreak, that even a relatively large initial number of cases could be dealt with."

Other experts say the nation is generally not yet prepared to deal with a massive biological weapons attack.

"We have seen how much suffering and disruption ensued from 18 cases of anthrax-a treatable disease. In the absence of significant improvements in our public health infrastructure, the country is vulnerable to the potentially calamitous consequences of a large bioterrorist attack," said Tara O'Toole, director of the Center for Civilian Biodefense Strategies at Johns Hopkins University in testimony earlier this year. O'Toole has praised significant increases in funding passed by Congress in the fiscal 2002 and 2003 appropriations bills.

Mississippi State Health Officer Ed Thompson who was involved in preparing the panel recommendation, acknowledged the panel had done no such analysis of response capabilities.

"What you were describing was a massive task that would take an enormous length of time," he said in an interview.

"We're having to make these decisions and recommendations based on a lot less information than we would like to have. Almost every decision that we've made with respect to bioterrorism out of necessity has been made with too little information," he said.

Other officials say there was no problem with a lack of analysis, and that the issue of readiness was discussed. "We've got more models than we could possibly imagine," said Henderson.

The question "was of course extensively discussed by the committee," says Modlin, a professor of pediatrics and medicine at Dartmouth Medical School. "It's one of those areas where there inevitably are more questions than there are answers."

Modlin said the committee did have the benefit of a CDC draft technical assessment of what would likely happen if an attack did occur.

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